1. Field of the Invention
This invention relates to improvements in methods for starting polyphase motors, and more particularly to improvements for starting polyphase motors of which the initial position and starting direction are not known, or need to be controlled.
2. Description of the Prior Art
In many instances, the startup of a motor needs to be controlled. For example, in personal and other computers, disk drives, such as hard disk, so-called "floppy", and other disk drives, polyphase, brushless DC motors are frequently used. In such applications, the startup of the disk drive motor needs careful attention. With increased emphasis on the speed of operation of modern computers, it is, of course, desirable to bring the disk drive motor, and the disk (or disks, or other load) driven by the motor, up to operating speed as rapidly as possible. Two of the problems which need to be addressed are the determination of the initial position of the motor to enable a voltage to be applied which will result in maximum startup torque, and the determination of the direction of rotation in which the motor will start. However, in many recent disk drive designs, polyphase, typically 3-phase, brushless DC motors which have been finding widespread use, generally have no controls to insure the rotor stops at any particular position. It can therefore be seen that it would be desirable to determine the position of the rotor of the motor upon startup to determine the how a startup voltage should be applied to the stator windings to result in maximum startup torque to bring the motor, and the disk rotated by the motor, up to operating speed as rapidly as possible.
Typically, recent polyphase, 3-phase, brushless DC motors used in disk drives and sundry other systems, have a plurality of stator windings located in the interior portion of the motor, and a cylindrically shaped rotor which carries a plurality of permanent magnets which interact with the fields generated by the magnetic fields of the stator windings to produce a rotating torque to turn the rotor. Although the motor is referred to as a three phase motor, in practice a DC potential is sequentially connected to successive stator windings to provide the magnetic fields to interact with the magnetic fields of the rotor magnets to produce the desired rotation of the rotor.
One of the problems associated with such motor is that generally after operation the rotor stopping position is not known, and to restart the motor by the application of startup voltage in a random fashion may tend to initially start the motor in the wrong direction. This can be a major problem, especially, for instance in magnetic disk applications, in which large stiction forces may exist between the disk surface and the disk head, especially if the disk has been allowed to remain idle for extended periods of time. Such stiction forces can result in the head being moved in a backward direction, risking damage to both the disk and the head and drive components.
In the past, one way motors have been started is merely by a "start and go" method in which the stator coils are energized and allowed to carry the rotor up to speed without regard either to the initial phase of the voltage applied to the stator of the motor or to the initial position or rotation direction of the rotor. (It is noted that even if the motor does initially start rotation in the wrong direction, the motor stator field will always rotate in the correct direction, and will eventually correct the direction of rotation of the rotor; but not without risking possible component and disk damage in the meanwhile, as mentioned above.) Thus, the startup torque to which the motor started in this way experiences is fortuitous, depending upon the particular, random relationship between the rotor and the stator fields. The motor, therefore, is not necessarily brought up to operating speed as rapidly as possible, resulting in a slower system operation than might otherwise be possible.
Another method which has been used is a "brute force" method in which a known fixed (i.e., non-rotating) field is applied by the stator to force the rotor to a known position before the phase is allowed to change, or the operating voltages switched to the next phase, to begin the desired rotation. Techniques of this type make no provision for the initial direction of rotation of the rotor, and, in worst case, would require rotation of the rotor from a position 180.degree. out of phase with respect to the desired starting position.
Another technique which has been used is to dynamically determine the position of the rotor as its rotation is begun. In such technique, typically the back emf induced in the stator coils upon rotation is measured, and the phase of the operating voltage adjusted, for example, by pulse width modulation or other techniques. However, since the back emf is directly proportional to the rotational velocity of the rotor, upon initial startup, the back emf is very small and difficult to reliably detect. And, of course, this technique requires rotation to enable its operation; consequently, the direction related problems mentioned above exist. Finally, as attempts are made to digitize computer operations to as large an extent as possible, generally the back emf is measured using digital techniques. But, as mentioned, especially at startup, the small signals are difficult to detect, and this adds additional concerns to digital signal processing.
Another technique which has been used is to apply a short burst of current to each set of stator coils, and measuring the amplitude of the current response. From this data, one of the current responses will be larger than the rest, enabling a stator coil to be determined to which a longer current pulse can be applied to start the motor in a desired manner. This process of applying pulse sets is continued until the motor reaches a predetermined RPM, after which other running techniques are employed. Such pulse set techniques, however, require wait times, due to data processing and the startup procedures themselves, and, therefore, result in less than maximum startup torque being achieved.